The present invention relates to improvements in vacuum frames and exposure devices for light sensitive material. Numerous shortcomings in prior art devices have existed and the present invention addresses various such aspects to improve upon the functioning of the vacuum frame.
A vacuum frame consists generally of a support in which a flexible blanket and light-transmissive plate form two major boundaries of a flat chamber. A negative and light sensitive material are placed on the blanket, so that light passing through the light-transmissive material will be modified by the negative and expose the light sensitive material. When the flat chamber is closed, a vacuum is drawn in that chamber so that the blanket will flexibly be urged against the glass and bring the negative and light sensitive material into intimate contact. A particularly preferred vacuum frame arrangement is disclosed in U.S. Pat. No. 4,754,309 issued to assignee of the present invention on Jul. 25, 1986. The entire disclosure of that patent is hereby incorporated herein by reference.
The light sensitive materials to be exposed come from various manufacturers who develop new light sensitive emulsions quite regularly. Each emulsion has its own sensitivity to light, so that the wave length of the light impacting on the light sensitive material and the relative intensity at the various wave lengths of the spectrum are critical to getting an optimum exposure. Also, there are various light sources, which can be used, and each has its own spectral output, which may vary according to operating conditions.
In order to achieve good exposures, it is important to match up the spectral output of the light source with the sensitivity of the light sensitive material, and to insure that the exposures are controlled so that the desired correlation is obtained and maintained. A frequently used light source is a high intensity discharge light in which an envelope of glass is provided with electrodes on either end and filled with a gas, and sometimes also dopant materials. When a very high voltage is applied across the two electrodes of the light source, the gas in the envelope ionizes and becomes quite conductive so that electrical current is maintained between the two electrodes. As the lamp gets hot, it begins to glow and the dopant vaporizes and ionizes. The current passing through the gas and dopant causes the materials to emit light, with the frequency and wavelength of the light being determined by the temperature of the envelope. The desired operating temperature for these bulbs is typically on the order to 900 degrees Celsius or greater, in order to obtain the spectral distribution from the lamp as contemplated by its manufacturer.
As can be appreciated, the lamp does not immediately jump to such a high temperature from room temperature at start-up, but rather takes some time to reach its optimum operating temperature. As such, it is desirable to maintain the lamp in a standby condition very close to its operating temperature, so that when an exposure is desired, it is not necessary to wait for lamp warmup. In addition such lamps are difficult or impossible to make conducting when they are hot, so turning them off requires a cool-down period on the order of several minutes before they can be turned on again.
Such lamps are reliable sources of their designed spectrums, as long as the lamp power supply is supplied with electrical power of a constant voltage. However in industrial settings, where devices of this nature are used, there can be wide swings in voltage applied to lamps from other loads on the electrical system, causing voltage surges and drops. These can materially adversely effect the control that the operator has over the light output from the lamp. Also, as the lamp ages, its operating characteristic can vary, presenting an additional variable which it is desirable to control.
Prior art devices have attempted to accommodate the voltage fluctuation and lamp life deterioration problems using integrators which integrate either the voltage applied to the lamp power supply or the measured light output from the lamp, as measured by a photocell. Both of these approaches have merit, but neither have in the past been accomplished in a manner which permits maximum control over the exposure, in order to achieve precise results.
Furthermore, prior art integrators have used only one means of monitoring the lamp operation in order to determine when to stop directing light from the lamp to the light sensitive material. This results in a lack of flexibility and lack of precision control over the light output.
In addition, problems have been encountered with lamps, particularly in their replacement in that workers sometimes prematurely try to handle a hot lamp when replacing it. They can be severely burned by the extraordinarily hot lamp, or subjected to dangerous electrical shock from touching the electrical connections while attempting to change lamps.
Also, the power applied to the lamp has in the past been selectable, at most, three increments as dictated by switching of inductive ballasts in series with the lamp circuit. This provides less control over the light output than is optimum.
The light output from the HID lamps is often in the ultraviolet range and is useful for exposing the light sensitive materials sensitive to such high energy wavelengths. However, other distributions of light output are also desirable, depending upon the results sought. Filters have been known to modify distribution of wavelengths from the light source but have had only limited utility in prior art devices.
Another shortcoming of the prior art devices has been in assembling them on a customer's premises. The various components from an integrator or other control device to a vacuum control, to the light source or the like have been difficult and problematic to wire. As can be appreciated, if the wire intended to go to the light is instead hooked up to the vacuum, calamitous results may occur.
Also, the operation of the lamp, as stated before, is very temperature sensitive. Temperature fluctuations can change the spectral output, so a lack of control over lamp temperature can lead to undesirable exposures. Also, as the lamps age, they can develop hot spots or cold spots where the dopants preferentially have condensed on parts of the envelope and do not re-vaporize as readily. These problems also require resolution.
In addition, the worker who mans a vacuum frame for extended periods of time has a tedious job, which can lead to inattentiveness. With so much care being required to obtain just the right exposure, there is a need to provide the operator with an easy way to cause the lamp and vacuum frame to operate properly, even if the worker becomes distracted.
Accordingly, as can be seen, there are numerous shortcomings in prior art devices of this sort needing resolution.